1. Field of the Invention
The present invention relates to a plasma torch and a method for adjusting the electrode position in the torch.
2. Description of the Related Art
In a plasma torch, the main arc used in the welding process is struck between the torch center electrode and the work. The nozzle section of the torch is comprised of two concentric chambers. The inner chamber houses a tungsten electrode, and the chamber lower end is provided with an orifice which is concentrically aligned about the electrode tip. The plasma gas is fed into this chamber. The inner chamber is surrounded by another chamber, which exits concentrically surrounding the orifice of the inner chamber. The inert gas forming a protective gas shield about the arc is fed into this outer chamber.
Since the transferred arc of the plasma torch needs to be struck in a gas atmosphere between the work and the center electrode, the plasma gas must be ionized prior to the initiation of the main arc to make the gas capable of supporting electrical conduction. Ionization is effected by means of a pilot arc struck between the center electrode and the nozzle piece forming the inner chamber. The pilot arc ionizes the plasma gas, whereby a conductive path of ionized gas is formed between the work and the electrode, thus facilitating the initiation of the main arc. The pilot arc can be struck by moving the center electrode to touch the nozzle piece, whereby an electric circuit is established.
The transferred main arc should be maintained between the center electrode and the work only, because the nozzle will be rapidly destroyed if double-striking moves the high-energy main arc to strike between the electrode and the nozzle. Hence, the electrode tip must be aligned precisely to the electrical center of the nozzle orifice. If the nozzle orifice and the electrode tip are fully symmetrical, the electrical center is generally also coincident with the geometrical center of the nozzle.
In a great number of torch constructions intended for manual welding, the electrode position can be altered only by machining the electrode tip, because the electrode is centered in a fixed manner by means of ceramic support pieces fitted in the interior of the nozzle. Such regrinding of the electrode is a slow and time-consuming operation inasmuch as machining tools must be used due to the stringent requirements of a high-precision tip shape.
In large-size mechanically controlled plasma torches used for welding, electrode centering can be made by means of an eccentric mechanism. This kind of torch has a large-diameter nozzle orifice and the main arc is initiated with the help of a high-frequency arc struck in a rotating manner between the center electrode and the surrounding nozzle piece in the annular gap of the nozzle orifice. Centering of the electrode is effected by first initiating the high-frequency arc and then moving the electrode by means of the eccentric mechanism until the arc is seen to rotate symmetrically about the annular gap of the nozzle orifice. However, such a mechanism is excessively robust for manual torches, and therefore, can be used only in torches equipped with high-frequency initiation. As the torch is not gas-tight during the centering of the electrode, this arrangement does not permit adjustment of electrode position when the main arc is running stable.
In U.S. Pat. No. 5,208,442 is disclosed a plasma torch in which the electrode is mounted to the torch body by means of a tightenable ball-and-socket joint, whereby the electrode can be pivotally rotated supported by the joint for the adjustment of the electrode tip position by means of a knob located in the upper section of the torch, after which the electrode may be locked in place by tightening the joint. While this device serves well in manual welding, the welding machinery must be halted in automatic welding when the adjustment of electrode position is necessary. Furthermore, manual control cannot be used for adjusting the plasma jet intensity (jet size) during welding, which results in a work-slowing drawback in certain types of welds. Particularly the start and finishing steps of single-pass through-welded pipe seams and other welds of circular joints may easily cause defects in the weld.
It is an object of the present invention to provide an assembly permitting automatic adjustment of the electrode tip position in a plasma torch when required.